(1) Field of the Invention
The present invention is directed to a sensing and tracking system and method of use for enhancing laser based acousto-optic sensing with the sensing and tracking system employing tailored retro-reflector devices. The retro-reflector devices amplify acoustics sensed in particular frequency bands as a function of an acoustic incidence angle and a laser beam interrogation angle. The sensing and tracking system searches for optical reflections or glint from one or more deployed retro-reflector devices in order to measure surface vibrations of the devices. The surface vibrations are caused by the amplified acoustics of an underwater sound source incident upon the devices.
2) Description of the Prior Art
A laser Doppler vibrometer (LDV) is a commonly-known device which is capable of directing a single output laser beam onto a measuring surface in which surface vibrations are measured when the laser beam is reflected back from the measurement surface into the LDV. Interference between the output laser beam and the reflected beam provides a measurement of the velocity and displacement of the surface. This action is useful for measuring surface vibrations produced by pressure waves from an underwater source.
However, reflections from a water surface tend to be weak, sporadic, and thus difficult to detect. A surface with poor reflective quality, turbulence, a high sea state or a foamy condition degrades sensor performance by defecting the laser bean away from the detector; thereby, increasing signal dropout. One remedy is to monitor the water surface glint locations and to continually steer the laser beam onto an optical glint—such as a naturally direct reflecting point on the unaided water surface.
Glint detection is accomplished by directing a laser at the water surface; identifying areas of direct reflection back to the source; obtaining an image of these temporally and spatially varying laser glint positions using a photosensor array; and steering the beam into the glint location on the water surface. In a real world example, laser based tracking systems are employed during eye surgery to accommodate eye movement during the operation.
A combined LDV and tracker system finds the points where reflection will occur by employing a complex algorithm to continually steer the light beam of the tracker in order to maintain sight of the dynamic reflection point on the water surface. An issue with this system is that natural water surface glints depend on sea state wave slopes and predominantly occur at a nadir +/−20°. This limits the angles at which the laser can be employed to probe the water surface.
Since the water surface acts as a specular reflector; the output laser of the LDV must be perpendicularly incident to the water surface in order to acquire a reflected beam. In normal operating situations, turbulent and hydrodynamic conditions prevail; thereby, resulting in significant intermittence of a received optical signal.
Intermittance of signal detection occurs when the slope of the wave surface changes with respect to the incident laser beam angle. The most troublesome problem governing LDV performance on moving reflective surfaces is signal dropout. In such a situation, it is difficult for the sensor system to capture the reflected beam.
Another problem is the poor reflective quality of the surface. One remedy is to illuminate the surface and to track the reflections. Image-based tracking using an array of photosensors finds points where the required reflection will occur in the tracker which seeks to find the point of reflection and which relies on complicated algorithms to steer the laser beam onto this dynamic point.
Natural water reflections or glints points that are identified by an illuminating source, predominantly occur at the nadir +/−20°. This occurrence limits the angles that the laser output beam may probe the water surface. Also, glint features on the water surface tend to be temporarily and spatially indeterminate which also makes continuous tracking difficult.
An example of a system for tracking glints is disclosed in U.S. Pat. No. 7,251,196, (Antonelli et. al.) and is commonly assigned to the assignee herein. The teachings of U.S. Pat. No. 7,251,196 are incorporated herein by reference.
The Antonelli reference discloses a passive acoustic sensor to detect underwater sounds using optics in order to determine vibration on the unaided surface. The tracking system must produce a light beam that is perpendicularly incident to the water surface. This positioning reduces the angular approaches that are available for tracking.
In addition, the turbulent and hydrodynamic wave conditions that often prevail under normal situations cause the slope of the turbulent wave surface to rapidly change. Accordingly, significant intermittence or signal dropout of the optical signal reflected from the water surface is expected as the slope changes relative to the incident angle of the laser beam.
A known laser-pumped acoustic sensor system is disclosed in U.S. Pat. No. 7,113,447 (Matthews et al.) and assigned to the assignee herein. The teachings of U.S. Pat. No. 7,113,447 are incorporated herein by reference. The system of the cited reference discloses a laser-pumped compact acoustic sensor system, wherein one or more hollow spherical shells vibrate in response to impinging acoustic signals. The shells have one or more portions that are reflective of impinging laser radiation. A resilient matrix, in contact with the water, supports the shell.
Further, in the cited reference, a laser Doppler velocimeter transmits radiation onto the reflective portion of the shell and receives reflected radiation therefrom. The reflected radiation produces signals in the laser Doppler velocimeter that claim to be representative of acoustic signals in the water. A computer, responsive to the signals, produces a display representative of direction and range to a target.
Known optical sensor systems rely on reflections from a reflecting object. However, the reflections are not directional; meaning that the diffuse reflections reflect energy in all directions and only a small portion of the reflected energy is detected. Accordingly, these systems are susceptible to background light noise; thereby resulting in reduced reliability.
There is therefore a need for a system to amplify, locate, track and detect laser reflections with increased temporal and spatial resolution. Such a system should be able to track and detect reflections over broader interrogation angles; detect reflections with reduced or no significant signal dropout; and increase the signal strength of reflected signals.